Density Functional Theory The Basis of Most Modern ... · The basis of most modern calculations...

Density Functional Theory The Basis of Most Modern Calculations

Richard M. Martin – UIUC

Lecture at Summer SchoolHands-on introduction to Electronic Structure

Materials Computation CenterUniversity of Illinois – June, 2005

Hohenberg-Kohn; Kohn-Sham – 1965Defined a new approach to the

many-body interacting electron problem

Reference: Electronic Structure: Basic Theory and Practical Methods,

Richard M. Martin (Cambridge University Press, 2004)R. Martin - Hands-on Introduction to Electronic Structure - DFT - 6/2005 1

electrons in an external potentialInteracting

R. Martin - Hands-on Introduction to Electronic Structure - DFT - 6/2005 2

The basis of most modern calculationsDensity Functional Theory (DFT)

• Hohenberg-Kohn (1964)

• All properties of the many-body system are determined by the ground state density n0(r)

• Each property is a functional of the ground state density n0(r) which is written as f [n0]

• A functional f [n0] maps a function to a result: n0(r) → fR. Martin - Hands-on Introduction to Electronic Structure - DFT - 6/2005 4

The Hohenberg-Kohn Theorems

n0(r) → Vext(r) (except for constant)


The Hohenberg-Kohn Theorems

Minimizing E[n] for a given Vext(r) → n0(r) and EIn principle, one can find all other properties and they are functionals of n0(r).



The Hohenberg-Kohn Theorems - Proof

The Hohenberg-Kohn Theorems - Continued

• Generalization by Levy and Lieb– Recast as a two step process

• Consider all many-body wavefunctions Ψ with the same density• First, minimize for a given density n• Next, minimize n to find density with lowest energy n0

• What is accomplished by the Hohenberg-Kohn theorems?

• Existence proofs

• A Nobel prize for this???• The genius is the next step –

to realize that this provides a new way to approach the many-body problem


The Kohn-Sham Ansatz

• Kohn-Sham (1965) – Replace original many-body problem with an independent electron problem – that can be solved!

• The ground state density is required to be the same as the exact density

• Only the ground state density and energy are required to be the same as in the original many-body system


The Kohn-Sham Ansatz II

• From Hohenberg-Kohn the ground state energy is a functional of the density E0[n], minimum at n = n0

• From Kohn-Sham

Exchange-CorrelationFunctional – Exact theorybut unknown functional!

Equations for independentparticles - soluble

• The new paradigm – find useful, approximate functionalsR. Martin - Hands-on Introduction to Electronic Structure - DFT - 6/2005 10

The Kohn-Sham Ansatz III

• Approximations to the functional Exc[n]• Requires information on the many-body system of

interacting electrons• Local Density Approximation - LDA

• Assume the functional is the same as a model problem –the homogeneous electron gas

• Exc has been calculated as a function of densityusing quantum Monte Carlo methods (Ceperley & Alder)

• Gradient approximations - GGA• Various theoretical improvements for electron density

that is varies in space R. Martin - Hands-on Introduction to Electronic Structure - DFT - 6/2005 11

What is Exc[n] ?

• Exchange and correlation → around each electron, other electrons tend to be excluded – “x-c hole”

• Excis the interaction of the electron with the “hole” –spherical average – attractive – Exc[n] < 0.


nx(r, r′)→

→ →

→

50

25

−0.2 −0.1 0.10(r′ − r)/o0

LD

Exact

nx(r, r′)

2.0

3.0

1.0

Exact

LD

−1.0 −0.5 0.0 0.5(r′ − r)/o0

rr = 0.09o0

r = 0.09o0

r = 0.4o0

r = 0.4o0

r′ − r r′ − r

+

+

0.05

0.10

0.15

Exact

0.0 0.5 1.0r′′

r′′ nx3.0 (r, r′′)

LD

(b)

(a)

Exactr′′ nx

5.0 (r, r′′)

r r′′

0.0 0.1 0.2 0.3 0.4

1.0

0.5

LD

r′′/on

Exchange hole in Ne atomFig. 7.2 Gunnarsson, et. al. [348]

Very non-spherical

Spherical average very closeto the hole in a homogeneouselectron gas!

Exchange-correlation (x-c) hole in silicon

• Calculated by Monte Carlo methods

1

(b)(a)

−0.1

−0.075−0.05−0.025

00.950.850.750.65

0.55

−0.1

0.75

1

0.5

Exchange Correlation

Fig. 7.3 - Hood, et. al. [349]

Hole is reasonably well localized near the electronSupports a local approximation


Exchange-correlation (x-c) hole in silicon

• Calculated by Monte Carlo methods

Exchange-correlation hole – spherical averageBond Center Interstitial position Comparison to scale

ρ xc(

r, R

)

(c )

0.0 2.0 4.0 6.0 8.0

R (a.u.)

VMCLDA

VMCLDA

−0.060

−0.040

−0.020

0.000

4.0 6.0 8.0

VMCLDA

0.0 2.0−0.060

−0.040

−0.020

0.000(a )

ρ xc(

r, R

)

10.04.0 6.0 8.0

VMCLDA

0.0 2.0

R (a.u.)R (a.u.)

(b )

−0.0040

−0.0030

−0.0020

−0.0010

0.0000

0.0010

ρ xc(

r, R

)

x-c hole close to that in the homogeneous gas in the most relevant regions of spaceSupports local density approximation ! Fig. 7.4 - Hood, et. al. [349]


The Kohn-Sham Equations

• Assuming a form for Exc[n]• Minimizing energy (with constraints) → Kohn-Sham Eqs.

Constraint – requiredExclusion principle forindependent particles

Eigenvalues are approximationto the energies to add or subtract electrons –electron bandsMore later



Solving Kohn-Sham Equations

• Structure, types of atoms

• Guess for input

• Solve KS Eqs.

• New Density and Potential

• Self-consistent?• Output:

– Total energy, force, stress, ...– Eigenvalues

Example of Results – Test Case

• Hydrogen molecules - using the LSDA(from O. Gunnarsson)


Calculations on Materials Molecules, Clusters, Solids, ….

• Basic problem - many electrons in the presence of the nuclei

• Core states – strongly bound to nuclei – atomic-like• Valence states – change in the material – determine

the bonding, electronic and optical properties, magnetism, …..


The Three Basic Methods for Modern Electronic Structure Calculations

• Localized orbitals– The intuitive appeal of atomic-like states– Simplest interpretation in tight-binding form– Gaussian basis widely used in chemistry– Numerical orbitals used in SIESTA

• Augmented methods– “Best of both worlds” – also most demanding– Requires matching inside and outside functions– Most general form – (L)APW

• Plane waves– The simplicity of Fourier Expansions– The speed of Fast Fourier Transforms– Requires smooth pseudopotentials


Plane Waves

• Kohn-ShamEquations in a crystal

• The most general approach

• Kohn-ShamEquations in a crystal

• The problem is the atoms! High Fourier components!R. Martin - Hands-on Introduction to Electronic Structure - DFT - 6/2005 20

Plane Waves• (L)APW method

• Augmentation: represent the wave function inside each sphere in spherical harmonics– “Best of both worlds”– But requires matching inside and outside functions– Most general form – can approach arbitrarily precision


Plane Waves• Pseudopotential Method – replace each potential

solid 2

Pseudopotential atom 1

• Generate Pseudopotential in atom (spherical) – use in solid• Pseudopotential can be constructed to be weak

– Can be chosen to be smooth– Solve Kohn-Sham equations in solid directly in Fourier space

1 2


Charge Density of Si – Experiment- LAPW calculations with LDA, GGA

• Electron density difference from sum of atoms– Experimental density from electron scattering– Calculations with two different functionals

• J. M. Zuo, P. Blaha, and K. Schwarz, J. Phys. Cond. Mat. 9, 7541 (1997).

– Very similar results with pseudopotentials• O. H. Nielsen and R. M. Martin (1995)

Exp LDA GGA


Comparisons – LAPW – PAW -- Pseudopotentials (VASP code)

• a – lattice constant; B – bulk modulus; m – magnetization

• aHolzwarth , et al.; bKresse & Joubert; cCho & Scheffler; dStizrude, et al.



Phase Transitions under PressureSilicon is a Metal for P > 110 GPa

• Demonstration that pseudopotentials are an accurate “ab initio” method for calculations of materials

• Results are close to experiment!– M. T. Yin and M. L. Cohen, Phys. Rev. B 26, 5668 (1982).– R. Biswas, R. M. Martin, R. J. Needs and O. H. Nielsen, Phys. Rev. B 30, 3210 (1982).

The Car-Parrinello Advance

• Car-Parrinello Method – 1985– Simultaneous solution of Kohn-Sham equations for electrons

and Newton’s equations for nuclei– Iterative update of wavefunctions - instead of diagonalization– FFTs instead of matrix operations – N lnN instead of N2 or N3

– Trace over occupied subspace to get total quantities (energy, forces, density, …) instead of eigenfunction calculations

– Feasible due to simplicity of the plane wave pseudopotentialmethod

• A revolution in the power of the methods– Relaxation of positions of nuclei to find structures– Simulations of solids and liquids with nuclei moving thermally– Reactions, . . .

• Stimulated further developments - VASP, ABINIT, SIESTA, . . .R. Martin - Hands-on Introduction to Electronic Structure - DFT - 6/2005 26

Simulation of Liquid Carbon

• Solid Line: Car-Parrinello plane wave pseudopotentialmethod (Galli, et al, 1989-90)

• Dashed Line: TB potential of Xu, et al (1992)

2 4 600.0

0.5

1.0

1.5

2.0

2.5

r (a.u.)

Rad

ial d

ensi

ty d

istri

butio

n g(

r)

“snapshot of liquid”


Example of Thermal Simulation

• Phase diagram of carbon• Full Density Functional “Car-Parrinello” simulation • G. Galli, et al (1989); M. Grumbach, et al. (1994)


Nitrogen under pressure – Recent discoveries

W. D. Mattson, S. Chiesa,R. M. Martin, PRL, 2004.

Squeezed&

Cooled•P > 100 Gpa and 0K•Network solid•Predicted > 15 years ago (DFT)•Found experimentally in 2000•New Prediction of Metallic N

•Hot Molecular Liquid•58 Gpa 7600 K•Nitrogen Molecules Disassociate and Reform

• Used SIESTA code for MD simulation• Sample structures tested using ABINIT


What about eigenvalues?• The only quantities that are supposed to be correct in the

Kohn-Sham approach are the density, energy, forces, ….• These are integrated quantities

– Density n(r ) = Σi |Ψi(r )|2– Energy Etot = Σi εi + F[n] – Force FI = - dEtot / dRI where RI = position of nucleus I

• What about the individual Ψi(r ) and εi ?– In a non-interacting system, εi are the energies to add and subtract

“Kohn-Sham-ons” – non-interacting “electrons”– In the real interacting many-electron system, energies to add and

subtract electrons are well-defined only at the Fermi energy

• The Kohn-Sham Ψi(r ) and εi are approximate functions- a starting point for meaningful many-body calculations


Electron Bands

• Understood since the 1920’s - independent electron theories predict that electrons form bands of allowed eigenvalues, withforbidden gaps • Established by experimentally for states near the Fermi energy


Extra added electronsgo in bottom of conduction band

Missing electrons(holes) go in top of

valence band

Empty Bands

Silicon

Gap

Filled Bands

Bands and the “Band Gap Problem”• Excitations are NOT well-predicted by

the “standard” LDA, GGA forms of DFTExample of Germanium

Ge is a metal in LDA!

L Γ χ W K Γ

−10Ge

−5

0

5

10

Ene

rgy

(eV

)

L Γ ∆ χΛ

Γ1

−10

L2′

L1Ge χ

1

−5

0

5

10

L2′

L3

L1

L3′

Γ15

Γ2'

Γ25′χ

4

χ1

Ene

rgy

(eV

)

Many-body theorywithExperimental points

LDA bands

Many-body bands

M. Rohlfing, et alR. Martin - Hands-on Introduction to Electronic Structure - DFT - 6/2005 32

The “Band Gap Problem”• Excitations are NOT well-predicted by

the “standard” LDA, GGA forms of DFTThe “Band Gap Problem”

Orbital dependent DFT is more complicated but gives improvements -treat exchange better, e.g,“Exact Exchange”

M. Staedele et al, PRL 79, 2089 (1997)

Ge is a metal in LDA!


Failures!• All approximate functionals fail at some point!• Most difficult cases

– Mott Insulators – often predicted to be metals – Metal-insulator Transitions– Strongly correlated magnetic systems– Transiton metal oxides – Hi-Tc materials – . . .


Conclusions I• Density functional theory is by far the most widely

applied “ab intio” method used in for “real materials” in physics, chemistry, materials science

• Approximate forms have proved to be very successful

• BUT there are failures• No one knows a feasible approximation valid for

all problems – especially for cases with strong electron-electron correlations


Conclusions II• Exciting arenas for theoretical predictions

– Working together with Experiments– Realistic simulations under real conditions– Molecules and clusters in solvents, . . .– Catalysis in real situations– Nanoscience and Nanotechnology– Biological problems

• Beware to understand what you are doing– Limitations of present DFT functionals– Care to use codes properly


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